This book is not a text on telephone systems. It is concerned rather, with the basic techniques of switching circuit design: techniques which are applicable to digital computers and other complex control systems as well as telephone switching systems. The wiring of this text ... started as a series of lecture notes for using in training new engineers in the Switching Systems Development Department of the Bell Telephone Laboratories. ...Arrangements were [later] made with M.I.T. to give graduate students a one semester course on the material used for training within the Laboratories.